This invention relates to turbocharged internal combustion engines, turbochargers for such engines, and to a method of preventing the turbine of a turbocharger from choking at high speed. More particularly, it relates to novel improvements to the so-called xe2x80x9cwastegate systemxe2x80x9d which, under certain operational conditions, directs some of the engine exhaust gas away from the turbocharger turbine inlet.
A turbocharger typically has a turbine driven by engine exhaust gas and a compressor for supplying compressed atmospheric air for engine combustion, both turbine and compressor being fixed to a common rotating shaft. For componentry of fixed dimensions, it is not possible to have a turbine, compressor and engine combination that is perfectly suited for all engine duties, load and speed ranges. Usually turbocharger components are matched to suit particular engine duty in order to give best engine/turbocharger overall efficiency at a chosen speed and load condition. Outside this envelope, the efficiency will be less.
Where a small turbocharger turbine is chosen to match low engine speed and high torque requirements, this has the consequence that, at high engine speed, the pressure of the air will be too high (for a given compressor size) and is likely to cause difficulties for the engine.
Conversely, when a larger turbine is chosen to match high engine speed requirements, the turbine may not develop sufficient power at low engine speed to produce sufficient compressed air to meet engine requirements at low speed and high torque.
In the case where a small turbocharger turbine is chosen, the xe2x80x9cwastegate systemxe2x80x9d is commonly adopted to reduce the production of excess compressed air at higher speeds. This is achieved by diverting some of the exhaust gas, at higher engine speeds, from the turbine inlet and reintroducing the diverted exhaust gas at a point downstream of the turbine. While this prevents turbine choking, it is clearly wasteful of some of the exhaust gas energy.
On the other hand, where a larger turbine is chosen, the problem of insufficient compressed air at low engine speed can be addressed by mechanisms which reduce the effective turbine nozzle capacity, thereby simulating the characteristic of a smaller turbine. U.K. Patent No. GB-A-2106191 is an example of the use of air pressure to achieve this effect, compressed air being dumped through a regulator valve into air distribution pipes supplying slots or orifices which are formed through the outer stator ring and open in a radial or substantially radial direction into the exhaust gas flow between the stator blades or vanes.
Other mechanical systems are known for varying the turbine nozzle capacity by repositioning nozzle blade angles on demand, thereby suiting engine speed and load requirements. Unfortunately, such systems suffer from both a cost penalty and from the unreliability associated with increased mechanical complexity.
According to one aspect of the invention, a turbocharged internal combustion engine has a turbocharger which comprises an axial flow turbine arranged to be driven by a flow of exhaust gas generated by the engine and an air compressor arranged to be driven by the turbine for supplying combustion air to the engine. The turbine has stator blades defining nozzles positioned upstream of rotor blades, and has a diverter valve operable to divert part of the exhaust gas flow into a turbine region defined between the nozzle throats and the leading edges of the rotor blades.
In this manner, the diverted exhaust gas is passed into a low pressure region of the turbine where it can do useful work in driving the rotor blades while at the same time avoiding choking the stator blades, thereby improving efficiency of the engine/turbocharger system.
According to another aspect of the invention, a turbocharger comprises an axial flow turbine and an air compressor, the turbine having stator blades defining nozzles positioned upstream of rotor blades, a casing defining primary and secondary inlets for exhaust gas, the primary inlet being arranged to admit exhaust gas into the turbine upstream with the stator blades, and the secondary inlet being positioned to conduct exhaust gas into a turbine region defined between the nozzle throats and leading edges of the rotor blades.
The invention has further features which relate to both a turbocharged internal combustion engine and to a turbocharger. Preferably, at least some of the stator blades define an internal passage for receiving diverted exhaust gas, and each passage is positioned to direct the diverted exhaust gas into the turbine region. In this case each passage is preferably positioned to discharge through a convex side of its stator blade. In this manner the diverted part of the exhaust gas flow is discharged from the low pressure sides of the stator blades. At least one hole is preferably formed through the convex side of each stator blade into its internal passage, and the holes are positioned to direct the diverted gas into the turbine region.
According to a further aspect of the invention, a method of preventing the turbine of a turbocharger from choking at high speed, comprises diverting some of the exhaust gas, produced by an associated internal combustion engine, into a low pressure region of the turbine.
The method preferably includes increasing the proportion of exhaust gases that are diverted as a function of turbine speed. In the case where the low pressure region is defined between stator blade nozzle throats and downstream leading edges of rotor blades, the method preferably includes introducing the diverted exhaust gas through convex surfaces of the stator blades.